Devices for quantitative detection of linear and rotary movements are known. Optical encoders are used to detect the rotation angle, a length and a direction of a rotary movement, or, respectively, linear movement of moving bodies. The primary components of such a device are the emitter system, a grid plate, normally a grid disk or a grid straight edge, and the detector system. The emitter system normally contains a light emitting diode (LED). The light beam emitted from the light emitting diode or laser diode is modulated by the grid plate. The grid plate is connected to a moving body and has a periodic opening pattern. The detector system detects the transmitter signal (modulated by the grid plate) from the laser diode and, at the output, supplies information relating to the light beam and the direction of movement.
High-resolution magnetic encoders using Hall sensors are also known. Furthermore, magnetic encoders (magneto-electric converters) that employ a magnetoresistance effect element made of a thin ferromagnetic film, have been commonly used in various fields due to their good durability in a surrounding atmosphere, wide operational temperature range, and high response frequency. For example, magnetic encoders are used for controlling the rotational speed of a capstan motor in a video tape recorder or the like. Generally speaking, magnetic encoders are used for positional or speed control in factory automation (FA) equipments, such as servomotors, robots and the like, or in office automation (OA) equipments, such as printers and copying machines. In recent years, there has been an increasing demand for improving the accuracy of such equipments. In general, the magnetic encoder includes a magnetic recorder and a magnetic sensor disposed in opposition to the magnetic recorder. The magnetic recorder comprises a non-magnetic substrate and a recording medium, which is a permanent magnetic material, coated on the peripheral or flat surface of the non-magnetic substrate. The recording medium is magnetized in a multipolar fashion at a magnetizing pitch λ to form at least one magnetic signal track.
A hard disk drive (HDD) is a digital data storage device that writes and reads data via magnetization changes of a magnetic storage disk along concentric information tracks. During operation of the HDD, the disk is rotated at speeds in the order of several thousand revolutions-per-minute (RPM) while digital information is written to or read from its surface by one or more magnetic transducers. To perform an access request, the HDD first positions the sensor and/or write head, also referred to as a “read/write head”, at the center of the specified data track of the rotating disk.
During operation of the HDD, the read/write head generally rides above the disk surface on a cushion of air, caused by an “air bearing surface,” that is created by the movement of the disk under the head. The distance between the read/write head and the disk surface while riding, or partially riding, on the air cushion is referred to as the “flying height” of the head. Further, the head is carried by a “slider” which is supported by hydrodynamic lift and sink forces. These lift forces are given by the interaction of air streaming underneath the surface structure of the slider.
To build encoder applications with high resolution, it is important to minimize the gap between the sensor and the information track. As the air bearing surface varies with the rotation speed, using air pressure as with the HDD applications is not possible if the variation of the relative movement is too high. Known optical encoders are limited to a small temperature range due to high sensitivity of the used sensors to temperature changes. The resolution of these encoders is also very sensitive to dust and humidity of the environment. Hall sensors are very sensitive to temperature changes and thus can also not be used in a wide temperature range as required in the field of automotive applications, industrial applications or the like.